As a near-field communication system, e.g., an IC (Integrated Circuit) system is widely known. In the IC card system, a reader/writer generates an electromagnetic wave, thereby generating a so-called RF (Radio Frequency) filed (magnetic field). An IC card is close to the reader/writer, then, the power is supplied to the IC card received by electromagnetic induction, and data is transferred between the reader and the writer.
The current specification of the IC card system includes a type A, a type B, and a type C.
The type A is used by Royal Philips Electronics as an MIFARE system. In the type A, data is encoded by Miller in the data transfer from the reader/writer to the IC card, and data is encoded by Manchester in the data transfer from the IC card to the reader and the writer. Further, the type A uses, as a data transfer rate, 106 kbps (kilo bit per second).
In the type B, data is encoded by NRZ in the data transfer from the reader/writer to the IC card, and the data is encoded by NRZ-L in the data transfer from the IC card to the reader/writer. Further, the type B uses, as a data transfer rate, 106 kbps.
The type C is used as a FeliCa system of Sony Corporation serving as the present applicant. Data is encoded by Manchester in the data transfer between the reader and the writer and the IC card. Further, the type C uses, as a data transfer rate, 212 kbps.
In the wireless communication such as near field communication, the problem of the so-called hidden terminal might be caused and therefore it is important to solve the problem.
For example, in a conventional wireless LAN (Local Area Network) system, generally, commands RTS (Request to send) and CTS (Clear to send) are received and sent in the data communication, thereby solving the problem of the hidden terminal (e.g., in non-patent document ANSI/IEEE Std 802.11, 1999 Edition, LOCAL AND METROPOLITAN AREA NETWORKS: WIRELESS LAN, Chapter 9 MAC sublayer functional description).
Here, the problem of the hidden terminal has the following problems.
That is, in the wireless communication, one of a plurality of communication devices sends data to another and then it is controlled that both the communication devices simultaneously do not output the electric wave (electromagnetic wave). Specifically, the communication device for outputting the electric wave detects the peripheral electric wave. In the case of detecting the peripheral electric wave, the communication device for outputting the electric wave does not output the electric wave. In the case of detecting no electric wave, the communication device for outputting the electric wave outputs the electromagnetic wave. Thus, the electric wave is alternately outputted between the one communication device and the other communication device for receiving and sending data.
When the communication device for outputting the electric wave controls the output of electric wave depending on the presence or absence of peripheral electromagnetic wave as mentioned above, one communication device might simultaneously send data to other communication devices and then the one communication device cannot receive the data.
That is, it is assumed that three communication devices A, B, and C exist. Then, the distance between the communication devices A and B is to control the exclusive use of electric wave therebetween. Further, the distance between the communication devices B and C is to control the exclusive use of electric wave therebetween. However, the distance between the communication devices A and C is not to control the exclusive use of electric wave therebetween.
In this case, the communication device B does not output the electric wave when any of the communication device A and the communication device C outputs the electric wave. However, the communication device A outputs the electric wave when the communication device C outputs the electric wave. Further, the communication device C outputs the electric wave when the communication device A outputs the electric wave.
When the communication devices A to C have the above-mentioned relationships, both the communication devices A and C might simultaneously send the electric wave (data) to the communication device B. For example, the distance between the communication devices B and A is equal to the distance between the communication devices B and C and both the communication devices A and C output the electric wave with the same strength, then, and the communication device B receives the individual electric waves outputted from the communication devices A and C with the same strength. Consequently, the crosstalk does not enable the normal reception of the data from both communication devices A and C.
As mentioned above, the communication device B does not normally receive the data because the communication device A confirms the existence of the communication device B and, however, does not confirm the existence of the communication device C, and the communication device C further confirms the existence of the communication device B and, however, does not confirm the existence of the communication device A. As mentioned above, the problem of the hidden terminal is that both the communication devices A and C are hidden from each other and are not viewed from each other and therefore the crosstalk is caused in the communication device B by simultaneously outputting the electric waves from both the communication devices A and C.
Then, in the conventional wireless LAN, the communication device on the communication source for starting the communication sends the command RTS for informing a communication time (time for sharing the space) to the communication device serving as the communication partner. The communication device, serving as the communication partner, for receiving the command RTS returns the command CTS for informing the acceptance for the command RTS and the communication time (time for sharing the space) to the communication device on the communication source. Other communication devices having the distance for receiving the command RTS or CTS from the communication device on the communication source or as the communication partner recognize the space sharing in one time for sharing the space in accordance with the command RTS or CTS, and do not send the electric wave (data) in the time for sharing the space.
In the communication devices A to C having the above-mentioned positional relationships, the communication device A sends the command RTS to the communication device B, and the communication device B sends the command CTS, serving as a response for the command RTS, to the communication device A. The communication device C can receive the command CTS sent by the communication device B and the communication device C receives the command CTS sent by the communication device B and then does not send the electric wave. Consequently, the communication device B prevents the collision of electric waves (data) from the communication devices A and C.
However, according to the solving method of the problem of the hidden terminal by using the commands RTS and CTS, the communication device needs control logic and memory therefore and thus costs are increased.